In general, press felts are used in papermaking machines to support the moist, freshly formed paper web as it encounters a variety of rolls which serve to extract water from the moist paper web. In addition to serving as a support for the paper web, the press felt serves as a receptacle for the water removed from the paper sheet. The press felt normally has a conveyor belt-like shape and during the various operations previously mentioned, a large amount of water is built up in the press felt which is removed by suction or various other drainage devices, usually after the paper web and press felt are no longer in direct contact. During the break-in or start-up period of most prior art felts, it is usually necessary to run the papermaking machine at a reduced speed and/or to increase the heat energy flowing into the dryer section. Since the amount of paper produced by a papermaking machine is largely dependent upon the speed of the machine, any slowing down caused by, for example, the required break-in period in papermakers' felts, results in the machine functioning at less than optimum economic efficiency.
Paper sheet dewatering, sheet surface smoothness, sheet pick-up and sheet carrying, as well as a variety of other performance characteristics, are sensitive to felt design. It has been observed that the physical characteristics of felts change during their useful operating life, the greatest change occurring during the first several hours of felt operation. Most press felts require an initial start-up period before peak water removal efficiency is achieved. This start-up period usually lasts for several hours and, in some cases, as much as several days. During the start-up period, press felt performance may be unsatisfactory in terms of optimum water removal from the paper web and pick-up and carrying of the paper web from the adjacent forming section or from an adjacent felt, thereby resulting in lost paper production and/or higher paper production costs due to lower operating speeds.
The subject of the present invention is a felt structure designed to substantially reduce or eliminate the problem of relatively poor paper machine performance during the early part of felt life. The new structure is based on the observation that the originally round or irregularly shaped felt surface fibers tend to flatten out during operation. As surface fibers flatten, the area of interface between felt and paper increases leading to greater adhesion between felt and sheet and therefore to improved sheet pick-up and sheet carrying performance. In addition, the increased sheet/felt interface area provides a more effective sheet support in the press leading to more efficient sheet dewatering. However, since fiber flattening under papermaking conditions requires time, during the early part of felt life the papermaker must do without the resultant benefits.